1. Field of the Invention
The present invention relates to an electrode for a light-emitting semiconductor device formed on a surface of a p-type GaN-base compound semiconductor, and to a method of producing the electrode.
2. Description of the Prior Art
In recent years, GaN-base compound semiconductor materials are drawing attention as a semiconductor material for light-emitting devices which emit short-wavelength light. The GaN-base compound semiconductor is formed on various oxide substrates such as sapphire single crystal or a III-V Group compound substrate by the metal organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy method (MBE method) or other such method.
A GaN-base compound semiconductor is a III-V Group compound semiconductor generally represented by AlxGayIn1-x-yN 0≦x≦1, 0≦y≦1, 0≦x+y≦1).
In the case of a light-emitting device formed by laminating layers of this GaN-base compound semiconductor that uses a substrate of an electrically insulating material, such as a sapphire substrate, an electrode cannot be provided on the back surface of the substrate, unlike when a semiconductor substrate such as a GaAs or GaP substrate is used that is electrically conductive. Accordingly, a pair of positive and negative electrodes are formed on the same surface of the light-emitting device. Also, when electricity is passed through the pair of electrodes to produce light emission, as the sapphire or other such substrate material is an insulator, the light is emitted from the surface on which the electrodes are provided. Namely, the light is emitted upward.
A characteristic of the GaN-base compound semiconductor material is that the current diffusion in the transverse direction is small. Due to this characteristic, even when electrodes are formed and light is emitted by passing electricity therebetween, the major part of the current flow takes place directly beneath the electrodes, as a result of which the light emission is limited to the region right under the electrodes and does not readily diffuse to the peripheral region of the electrodes. Therefore, in the case of conventional opaque electrodes, the light emission is interrupted by the electrode itself and cannot be taken out from the upside of the electrode. As a result, the intended improvement in the light emission intensity is not achieved.
To overcome this drawback, JP-A-6-314822 discloses a technique relating to the device structure whereby a light-permeable electrode comprising a very thin metal is used as a p-type electrode and formed almost over the entire front surface of the device to thereby allow the emitted light to pass through the light-permeable electrode and be emitted externally from the upper side. In this disclosure, Au, Ni, Pt, In, Cr, or Ti, for example, is used as the electrode material and the metal film formed by vapor deposition is heat-treated at a temperature of 500° C. or higher to induce sublimation of the metal, so that the thickness is reduced to from 0.001 to 1 μm to thereby impart light permeability. The term “light-permeable”as used herein with reference to the electrode refers to an electrode through which light emission generated under the electrode can be observed. To enable observation to take place through the electrode, the electrode must have a light transmittance of at least 10%.
However, such a thin metal film has low strength that makes it impossible to directly bond wires to the thin film for injecting electrical current from an outside source. For this reason, electrodes for use in semiconductor light-emitting devices generally employ a structure comprising forming, in addition to the light-permeable electrode, a wire-bonding electrode having electrical contact with the light-permeable electrode, and using this wire-bonding electrode to connect the wire used to carry current to the light-permeable electrode.
When a light-permeable electrode is formed using thin metal film, as shown by FIG. 23, the structure generally used comprises forming the wire-bonding electrode 8 on the light-permeable electrode 7. However, with this structure it is difficult to ensure adhesion between the front surface of the light-permeable electrode 7 and the lower surface of the wire-bonding electrode 8, sometimes causing the wire-bonding electrode 8 to peel off during the electrode production process.
To overcome this, JP-A-7-94782 discloses a technique for improving bonding properties, illustrated by FIG. 24. In this arrangement, a window 70 is formed in the light-permeable electrode 7 via which the surface of the semiconductor 9 is exposed, the wire-bonding electrode 8 is formed on the window 70 to effect direct contact between the wire-bonding electrode 8 and the surface of the semiconductor 9.
In most cases a thick bonding film about 1 μm in thickness is used for the wire-bonding electrode as a way of absorbing the impact of the wire bonder. Because it is that thick, light permeability cannot be imparted to the wire-bonding electrode. This means that light emission occurring directly below the wire-bonding electrode, and therefore cannot be emitted to the outside. Thus, to achieve higher emission brightness, a structure is required whereby current is not injected into the semiconductor portion directly beneath the wire-bonding electrode, but flow instead to the light-permeable electrode.
JP-A-8-250768 discloses a technique whereby current does not flow to the region below the wire-bonding electrode. This is achieved by providing the semiconductor layers below the wire-bonding electrode with a high-resistance region by various methods such as by forming a silicon oxide layer, leaving a region that is not subjected to p-type formation treatment, using annealing or ion implantation and so forth. The high-resistance region prevents current flowing under the wire-bonding electrode, directing the current instead to the light-permeable electrode to thereby efficiently use the current.
However, in the disclosure of JP-A-8-250768, the structure providing the high-resistance region under the wire-bonding electrode requires the formation of silicon oxide layers and steps to increase the resistance of the semiconductor. Thus, the process is complicated and production takes long time. For example, in order to form silicon oxide layers, it is necessary to use photolithography to effect patterning, or plasma CVD processes and the like. Similarly, photolithography, ion implantation, annealing and other such processes have to be used to form a high-resistance semiconductor region. All these processes are complex time-consuming.
Also, when the above-described high-resistance region arrangement is to be applied to the configuration of the above JP-A-7-94782 in which the wire-bonding electrode 8 is provided on the window 70 (FIG. 24), the high-resistance region is formed in the semiconductor 9 beneath the wire-bonding electrode 8. This produces an arrangement in which the current has to flow from the peripheral portion 8a of the wire-bonding electrode 8 into the semiconductor 9, via the light-permeable electrode 7, generating light emission in the injection region 91. Since the peripheral portion 8a acts as a barrier to the generated light, the light emission cannot be taken out upward. The light emission is therefore wasted, reducing emission efficiency.
The object of the present invention is to provide an electrode for light-emitting semiconductor devices, that uses a simple structure that is able to securely block current flow under the wire-bonding electrode and can improve the light emission efficiency.